A cold cathode fluorescent light (CCFL) has been increasingly used as a back light source of a liquid crystal display monitor of a notebook PC and of a liquid crystal display (LCD) for use with, for example, a TV set. Such CCFL has substantially the same high efficiency and life as a usual hot cathode fluorescent light, without using a filament of the hot cathode fluorescent light.
In order to start up and operate the CCFL, a high ac voltage is required. For example, a startup voltage of about 1000 V and an operating voltage of about 600 V are required. These high ac voltages are generated from a dc power supply unit of, for example, a notebook PC and a liquid crystal TV set, using an inverter.
Conventionally, a Royer circuit has been used as an inverter for the CCFL. The Royer circuit comprises a saturable magnetic core transformer and control transistors, and is adapted to undergo a self-sustaining oscillation owing to the nonlinear permeability of the saturable magnetic core and the nonlinear current gain characteristic of the control transistors. The Royer circuit itself requires no external clock or driver circuit.
However, a Royer circuit is basically a constant-voltage inverter, which cannot maintain a constant output voltage if the input voltage thereto and/or the load current thereof varies. Hence, in order to maintain a constant input voltage to the Royer circuit, a regulator for supplying electric power to the Royer circuit is required. For this reason, besides the inverter utilizing a Royer circuit has low power inversion efficiency, it is difficult to miniaturize such inverter.
A CCFL inverter having improved power conversion efficiency has been disclosed (see for example Japanese Patent Early Publication H10-50489). This inverter comprises a first semiconductor switch connected in series with the primary winding of a transformer, a serially connected second semiconductor switch and a capacitor which are connected in parallel with the primary winding, and a coupling capacitor and a load connected in series with the secondary winding of the transformer. The first and second semiconductor switches are switched on and off by a control signal received from s control circuit to supply ac power to the load.
A full-bridge (often called H bridge) type CCFL inverter utilizing four semiconductor switches has been proposed (see for example U.S. Pat. No. 6,259,615). This inverter has a transformer having its primary winding connected to the output terminal of the full-bridge via a resonant capacitor connected in series with the primary winding. The load is connected to the secondary winding of the transformer. Of the four semiconductor switches constituting the full-bridge, a first set of two semiconductor switches establishes a current path in a first direction to the primary winding of the transformer and a second set of two semiconductor switches establishes a current path in a second direction to the primary winding. The control circuit provides the full-bridge semiconductor switches with control signals each having a fixed pulse width and a controlled relative position of the pulse, thereby regulating the power given to the load.
Conventional inverters utilizing Royer circuit are not only difficult to be miniaturized but also disadvantageous in that their conversion efficiencies are low. In the inverter as disclosed in the first referenced document, the load current and load voltage cannot be detected accurately, since the primary circuit of the transformer includes capacitors, and since control signals are formed based on the primary current. On the other hand, the inverter as disclosed in the second referenced document is configured to have a serially connected capacitor in the primary winding and control ON-OFF operation of the full-bridge semiconductor switches by control signals having a fixed pulse width and controlled relative pulse positions in regulating the power to be supplied to the load. Hence, the inverter has a structural limitation that the four semiconductor switches must be of the same conduction type. Moreover, the prior art inverters have difficulty to regulate the power supplied to the load over a wide range, especially for the purpose of dimmer control.
The invention, therefore, provides an inverter for generating from a dc power supply an ac voltage for driving a load, the inverter comprising a transformer having a primary winding that includes a semiconductor switch circuit in the form of a half-bridge or full-bridge, and a secondary winding connectable to a load. The invention is directed to an inverter capable of finely regulating the power supplied to a load by implementing means for controlling the semiconductor switch circuit through pulse-width modulation (PWM), performing zero-current switching of the switch circuit, and preventing penetration current from occurring.
The invention is also directed to an inverter capable of finely regulating power supplied to a load over a wide range, especially during dimmer control for example.